Multi-wavelength switches (MWS) based on MEMS micro-mirrors are used in dense wavelength division multiple (DWDM) fiberoptic systems to combine a plurality of wavelength channels onto a common optical fiber. Such MWS switches have ‘m’ input ports and a single common output port. In operation, the user can select in a reconfigurable manner, which of the wavelength channels at the ‘m’ input ports are coupled through to the common output port. This selection is accomplished by knowing the voltage to apply to the MEMS micro-mirror actuator so that light is optimally coupled from the selected input port to the output port.
The MWS switch is also be used as a variable attenuator to attenuate the individual wavelength channels in order to equalize power in all the wavelength channels in the multiplexed light signal. The attenuation of each wavelength channel is controlled by detuning the coupling of light from the input port to the output port from its optimum value to a desired attenuation value. At the factory, the electro-optic response (coupled power versus micro-mirror tilt voltage) of each MEMS micro-mirror is measured in order to provide calibration voltages that can be used in determining the required voltage needed to obtain the desired attenuation. In this way, the calibration voltages are used in the control of an MWS switch.
A conventional prior art approach to measuring attenuation in an optical device is to directly measure the input and output signals as shown in FIG. 1. In a drift detecting and compensating optical system 100 an optical switch 101 such as a 5×1 MWS or similar device receives a plurality of input signals 121-125, from which a single one is selected by a controller 104 using a switch control 115 for transmitting to an output 111. The input signals 121-125 and the output 111 are tapped at a suitable split ratio, typically 10%, to input tap lines 131-135 and 112, respectively, which are connected to input ports on a 1×6 optical switch 103. An output 106 of the optical switch 103 can be controlled to select a signal from one of the input ports in order to feed it to an Optical Channel Monitor (OCM) 105 or similar equipment for measuring signal characteristics such as optical power, wavelength etc.
One such example is given by Sparks et al. (U.S. Pat. No. 6,625,340: Optical switch attenuator, Nortel Networks Limited) who teach calibrating an optical switch such that a predetermined (micro-) mirror misalignment produces a predetermined attenuation, so that only a single indication of the optical signal power is necessary. Such a power measurement could be performed substantially upstream or downstream of the optical switch, at a different point within the network if the attenuation characteristics of any intervening components are known. Alternatively, both the input and the output optical signal to the switch could be measured in order to directly indicate the degree of the attenuation of the optical signal as it passes through the switch. This information could be used to provide a closed loop feedback control system to ensure that the desired degree of attenuation is achieved for each optical signal (or channel).
A problem with this approach for controlling the MWS is that the electro-optic response of micro-mirrors can drift during operation for a variety different reasons. Thus the original calibration voltages for providing the desired attenuation values become inaccurate with time.
In a MWS the input and output signals potentially contain many signal channels, some of which are redundant at the input. Therefore the power levels from these sources must be measured independently with an OCM. The cost of an OCM is relatively high, therefore in practice it has to be shared amongst all inputs using an additional optical switch. With such a system, direct measurement of attenuation is relatively straightforward. The output and input signals are measured separately and then the attenuation can easily be calculated.
One advantage of this approach is the input signal from a channel entering a network at a particular node (an ‘add’ channel) can be verified prior to being commissioned.
Apart from additional hardware costs, there are several technical disadvantages of this approach. The input and output signals are not measured simultaneously, therefore any change in input power level during the measurement will appear as a shift in attenuation level. Any drift in the tap ratio over time will be also appear as an attenuation change. For smaller tap ratios this can be a significant issue.
To overcome the drift problem, devices and methods have been devised for determining the channel optical power levels by dithering the MWS attenuation.
Besler et al. (U.S. Pat. No. 6,549,699), for example, use a processing unit 104 in FIG. 1 to apply an appropriate alternating (or “dither”) control signal 115 to a channel micro-mirror in optical switch 101, in superposition on the DC control signal for maintaining the channel micro-mirror at a particular pivoting position, thus stabilizing it against drift. Both the optical power level of the corresponding spectral channel and the rate of change in the optical power level (or the time derivative of the optical power level) at the micro-mirror's pivoting angle can thus be obtained. The rate of change in the optical power level is proportional to the slope of a calibrated or pre-measured coupling efficiency curve, and is therefore useful in locating a micro-mirror's pivoting angle corresponding to the measured optical power level. From the pivoting angle thus derived, the magnitude of the feedback control signal can be determined for applying to the channel micro-mirror, so as to achieve the desired coupling efficiency in a most effective manner.
While a method such as that disclosed by Besler et al. can provide feedback suitable for reducing drift, it introduces an optical power variation or dither on the output signal 111 of the optical switch 101. This can have detrimental effects on the performance of an optical transmission system in which a drift stabilized switch is used. The problem is exacerbated in cases where several such switches are cascaded in the system. Furthermore, Besler's method relies on the micro-mirror electro-optic characteristic remaining stable with respect to the calibration in both absolute optical power as well as the DC control signal 115.
Thus a need becomes apparent in the industry for a way to remove or mitigate the optical power variations or dither produced by optical switches similar to the one described above.
It is an object of instant disclosure to provide a remedy for the above problem by compensating for, or canceling the effects of, the aforesaid optical power variations or dither.
A further object of instant disclosure is to provide a remedy for the practical situation where the micro-mirror electro-optic characteristics drift with environmental changes and aging.